There are several reasons it may be desirable to lock in place a manhole cover or sewer grating. A lock acts as an impediment to vandals who might remove a cover to leave an open hole and create a dangerous situation. Such covers are generally heavy, being of a cast metal such as iron and a child may drop such a cover on his toes or otherwise cause harm to himself when trying to dislodge such a cover from its place. In this way an unlocked cover can pose a hazard. The possibility exists that an automobile tire may flip a cover from its place as the auto passes across the cover. An unlocked cover may thus become a projectile and can pose a danger to things in its path as it flies through the air.
A number of locking arrangements for these types of covers have been patented over the years. Examples are given in the following U.S. patents:
U.S. Pat. No. 3,455,059 issued Jul. 15, 1969 to Evans; PA1 U.S. Pat. No. 3,921,494 issued Nov. 25, 1975 to Coe; PA1 U.S. Pat. No. 4,075,796 issued Feb. 28, 1978 to Cuozzo; PA1 U.S. Pat. No. 4,523,407 issued Jun. 18, 1985 to Miller; PA1 U.S Pat. No. 4,763,449 issued Aug. 16, 1988 to Vigneron et al.; PA1 U.S. Pat. No. 4,840,514 issued Jun. 20, 1989 to Defrance et al.; PA1 U.S. Pat. No. 5,065,955 issued Oct. 15, 1991 to Spiess et al.; and PA1 U.S. Pat. No. 5,071,177 issued Dec. 10, 1991 to Spiess et al.
U.S. Pat. No. 1,458,391, issued in 1922 to Burton shows a manhole cover having two retractable locking bolts which engage the flange of a surrounding support ring to secure the cover and ring together. Each locking bolt is pivotally connected to a central rotating bar. A handle, accessible from the top side of the cover is used to rotate the bar to retract the bolts from the locking position so that the cover can be lifted out of place.
U.S. Pat. No. 2,363,567, issued in 1943 to Blakeman, describes a coverplate having a locking arrangement similar to that of Burton, but the arrangement is such that the locking bolts are biased by gravity into their locking positions.
U.S. Pat. No. 4,902,165, issued to Embree on Feb. 20, 1990, shows a locking arrangement related to those of Burton and Blakeman, but in this case, the bolts are biased into position by means of a spring.
U.S. Pat. No. 4,964,755, issued to Lewis et al. on Oct. 23, 1990, again shows a latching mechanism having a pair of lock rods, in this case the rods being pivotally connected to a central rotatable crank plate. A spring biases a central crank upwardly to hold the crank in a locking position and thereby prevent its rotation without the use of a key.
U.S. Pat. No. 5,082,392, issued to Marchese et al. on Jan. 21, 1992, also shows a similar locking arrangement. In this case the bolts are spring biased into their locking position.
The approaches taken in these latter five patents all rely on a central rotational mechanism for retracting bolts from their locking positions. If the central rotational mechanism is broken, retraction of the bolts thereby becomes impossible which might greatly increase the difficulty of removing the cover. Further, such covers are often exposed to the elements and such rotational mechanisms may become frozen because of dirt or corrosion or rust, etc. In such a situation, the mechanism itself may have to be circumvented in order to remove the cover, possibly resulting in destruction of the mechanism in which case it would need to be replaced.